Sculpting & Modelling > Sculpting & Modelling Questions

Baking on glass items like jars and vases

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Just wondering if you guys have any tips on how to go about baking polymer clay covered glass. Foolishly I didn't look it up before I tried a piece tonight, thankfully the clay was okay but the glass underneath was warp and broke (it was already cracked but I think it would have cracked anyway) I read this is due to the sudden temp changes. Is there anything I can do to prevent this in further pieces?

monkey ann:
I guess you could try putting the clay covered glass item in a cold oven and let it warm up gradually as the ovens gets up to temp.  I do this if I am baking food in a ceramic or glass dish.

Have used glass jam jars as bangle formers, no problems with these going into a hot oven, glass is quite thick though.

First off, I have to say I love your project - it is wonderful!

As for your technical question. If the glass has warped in anyway, that indicates it isn't glass. Or if it is glass, it might be covered with some type of plastic coating. Because glass itself will not warp at the temperatures we bake clay at. Glass could break and crack with thermal expansion, but it won't actually warp or distort shape.

So before covering anything in clay, you need to be sure it is actually glass and not a glass/acrylic combination or just very good imitation glass of some type.

Secondly, as Ann says - you can put the glass into a cold oven and slowly bring it up to temperature. Once the clay has baked, shut the oven off and allow the piece to cool slowly back down to room temp before removing it from the oven.
This will minimise any risk of thermal shock to the glass and also allow everything to contract at a slower rate.

Another thing you can do, if you're getting cracking or warping in the CLAY layer is to leave a little expansion gap somewhere, so the clay has room to "breathe" around the glass. This can be as simple as a small gap left at the seam where the clay wraps around the glass.


Thank you Emma!

Perhaps you are correct with it not being actual glass. I'm not sure if it did actually warp I thought it did because I could no long put the nozzle part back on the top but that could have just been due to the cracking.

I'll try what you both suggested with letting it slowly heat up and cool, should I adjust the baking time then since the clay will be sitting in the heat for longer? Like say if I would back it for twenty should I adjust so its only at full temp for like 15 or 10?

I'll try leaving the seam as well. The thing I used for a base was just a cheapish vinegar type container so perhaps it wasn't actually glass, plus it was already cracked which is why I just had it laying around to try.

I picked up a little glass mason type jar to try a second one so i'll do what you guys suggest and let you know how it turns out :D! Now if I can just conquer magnets and pins i'll be getting somewhere.

To me in the top photo it looks a lot like plastic, the way it is cracked and also becuase of the foggy surface, but so little is visable so i could be wrong.


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